The present invention generally relates to a receiver, and particularly relates to a radio receiver using a switching diversity reception system in which one of a plurality of antennas is chosen for reception of a signal. The choice is made based on which antenna can best receive a signal. Further, the present invention relates to a receiver suitable for use in a karaoke system (a sing-along audio or video system) including a wireless microphone.
Conventionally, a diversity reception system has been known for obtaining a good reception state by using a plurality of antennas (see Japanese Patent Unexamined Publication No. Sho. 63-252025). Examples of a diversity reception system include a switching diversity system, a selection diversity system, and the like. Referring to FIGS. 2 and 3, a receiver of the switching diversity system will be described by way of an example.
In an FM diversity receiver 100, first and second antennas 1A and 1B are provided at suitable positions as shown in FIG. 2. Output signals of the respective antennas 1A and 1B are selectively applied to a front-end 2 through a switch SW.sub.5 which operates in accordance with a switching control signal S.sub.SEL supplied from an antenna selection switching circuit (described below). The front-end 2 converts the input signal into an intermediate frequency signal. The intermediate frequency signal is supplied to an IF amplifier 3 where the intermediate frequency signal is subjected to band-amplification to a level not lower than a predetermined value. The AM component of the band-amplified signal is removed by the limiter function of the IF amplifier 3. The thus-processed signal is converted into an audio signal by a detection circuit DET 4 or the like in the succeeding stage.
On the other hand, a level detection circuit 14 generates a field intensity level signal S.sub.LV from the intermediate frequency signal of the IF amplifier 3, and supplies the signal Shy to an antenna selection switching circuit 5. Further, the level detection circuit 14 supplies a muting circuit 15 with a muting control MC for causing the muting circuit 15 to perform a muting operation for the purpose of preventing noise signals from being outputted as an audio signal when the level of an antenna input signal is low.
The antenna selection switching circuit 5 has an AGC amplifier 6, which is supplied with the field intensity level signal S.sub.LV from the level detection circuit 14 through a coupling capacitor 7. Noise signals contained in the field intensity level signal S.sub.LV are amplified and rectified by the AGC amplifier 6. The rectified signal is supplied to a monostable multivibrator MMV.sub.1 so as to drive the MMV.sub.1 when the level of the signal is not lower than a predetermined value. Then, after the signal has been delayed in a delay circuit 8, a switch SW.sub.1 is closed for a time (for example, a few msec) determined by the MMV.sub.1.
If noise signals are still present when the switch SW.sub.1 is being closed, the rectified component of the noise signals is supplied to a monostable multivibrator MMV.sub.2, is waveform-shaped, and is then outputted as a single pulse to a pulse generation circuit 9. The pulse generation circuit 9, triggered by the single pulse output signal of the MMV.sub.2, generates pulses A, B, C, D and E successively, as shown in FIG. 3. The pulse A is supplied to a switch SW.sub.2 so as to cause a first sample-and-hold circuit 10 to sample and hold the field intensity level of the signal received by the currently selected antenna, for example, the antenna 1B in the illustrated case of FIG. 2.
Next, when the pulse B is generated, the pulse B is supplied to a switch drive 13 so as to switch the connection of the antenna through the switch SW.sub.5. The pulse C is supplied to a switch SW.sub.3 so as to cause a second sample-and-hold circuit 11 to sample and hold the field intensity level of the signal received by the antenna selected after the switching operation by the pulse B. That is, in the illustrated case, the signal received by antenna 1A is sampled and held by sample-and-hold circuit 11. The respective output signals of the first and second sample-and-hold circuits 10 and 11 are supplied to a comparator circuit 12 so that the field intensity levels of the output signals are compared with each other to thus output a result of such comparison. Next, the pulse D is supplied to a switch SW.sub.4 so as to close the switch SW.sub.4 so that the result of the comparison outputted from the comparator circuit 12 is supplied to the switch drive 13.
As a result, in the case where the field intensity level sampled-and-held by the first sample-and-hold circuit 10 is higher than that sampled-and-held by the second sample-and-hold circuit 11, the switch drive 13 connects the switch SW.sub.5 to the antenna (the antenna 1B in this example) which was used before the last switching operation (performed by using pulse B) and connected to the first sample-and-hold circuit 10. In the contrary case where the field intensity level sampled-and-held by the first sample-and-hold circuit 10 is lower than that sampled-and-held by the second sample-and-hold circuit 11, the switch drive 13 maintains the state in which the switch SW.sub.5 is left connected to the antenna (the antenna 1A in this example) used after the last switching operation (performed by using pulse B). Thereafter, the contents of the sample-and-hold circuits 10 and 11 are released in response to the pulse E so as to be ready to cope with the next occurrence of noise.
Therefore, the output signal levels of the antennas 1A and 1B are compared with each other every time noise signals are supplied to the MMV.sub.2 so that an antenna having a high field intensity level is always selected. In other words, the output signal levels of both the antennas are compared with each other every time the rectified component of a noise signal is supplied to the MMV.sub.2, and a higher-level antenna is always selected.
In the above-mentioned conventional receiver, the setting of the antenna switching operating point by the antenna selection switching circuit 5 is made by adjusting the gain of the AGC amplifier 6 so as to accord with the operating point of the muting circuit 15. Depending on the operating temperature characteristic of the AGC amplifier 6 or that of the muting circuit 15, or depending on the adjusted state of the AGC amplifier 6 or that of the muting circuit 15, however, there sometimes occurs a disagreement between the antenna switching operating point and the muting operating point. When the antenna switching operating point P.sub.1 is higher than the muting operating point M (by about 20 dB) as shown in FIG. 4, there occurs a disadvantage that a switching noise signal is present in an audio output signal at the time of antenna switching because the antenna switching is performed in a range of the field intensity level from P.sub.1 to M where the muting circuit does not yet operate. Furthermore, when the muting operating point M is higher than the antenna switching operating point P.sub.2, there occurs disadvantageously a so-called dead point, that is, a state where antenna switching is not performed in a range of the field intensity level from M to P.sub.2 although the muting circuit is operating.